On a matter related to several other current threads here, can anyone tell me
whether there are experiments that show that the time delay between the emission
and the (more or less remote) absorbtion of that exact same photon are
compatible with its speed being c ?

On a matter related to several other current threads here, can anyone tell me
whether there are experiments that show that the time delay between the emission
and the (more or less remote) absorbtion of that exact same photon are
compatible with its speed being c ?

An underlying assumption under this question seems that you consider
photons as point particles whose location, as well as times of
absorption and emission, can be measured with any desired precision.
That's a misguided view of what a photon is, to say the least. What is
understood by the word "photon" in the physics literature posesses none
of these properties. Take a look at this old post for a description of
what theorists and experimentalists mean by the word photon.

The fact of the matter is that as soon as you decide to measure
properties of individual photons, you should be prepared to wait a long
time for the measurement to finish as well as accept a large
uncertainty in its position, which makes measuring a photon's speed
ambiguous. If you are indeed interested in the speed of propagation of
the electromagnetic field (or light), then you have to consider either
wave fronts or pulses. However, neither of these configurations
possesses definite photon number. But, yes, there've been lots of
experiments done that have measured the speed of wave fronts and
pulses. And, yes, they are consistent with the speed of light in vacuum
being the limiting velocity.

On a matter related to several other current threads here, can anyone tell me
whether there are experiments that show that the time delay between the emission
and the (more or less remote) absorbtion of that exact same photon are
compatible with its speed being c ?

An underlying assumption under this question seems that you consider
photons as point particles whose location, as well as times of
absorption and emission, can be measured with any desired precision.

I'm not really assuming that. I understand that there are processes which
generate photons in pairs with opposite attributes like momentum etc. By
detecting one of these locally it ought to be possible to determine the time of
emission with a reasonable accuracy cf the time of flight. I also understand
that it is possible to detect single photons with eg ccd detectors, and it ought
also to be possible to reduce the uncertainty in detection time to be small cf a
long enough time of flight. I just wondered if there have been any experiments
along these lines that actually attempt to time transit times for individual
emmission / absorption events of photons ?

On a matter related to several other current threads here, can anyone tell me
whether there are experiments that show that the time delay between the emission
and the (more or less remote) absorbtion of that exact same photon are
compatible with its speed being c ?

Thanks,

--
Boo

Creating a device which produces exactly one photon on demand (a.k.a. a
photon gun) is quite a challenge and I'm not sure one has yet been
created. So the short answer is "no".

A further point is that within quantum theory the concept of "the exact
same photon" is not well defined. One should not think of a photon as
an identifiable object as such (no labels). Rather view photon
emission and absorption as quantized *phenomena* which causally
propagate at speed c. I.e. the emission of a photon can be said to
cause a later absorption of a photon. (always *a* never *the* and
never ever *the same*).

But finally given that the pulses generally measured at speed c are
ensembles of photons and given photons do not interact with one another
then the fact that variations in the beam propagates at exactly c in
vacuum then the photons must likewise.

On a matter related to several other current threads here, can anyone tell
me
whether there are experiments that show that the time delay between the
emission
and the (more or less remote) absorbtion of that exact same photon are
compatible with its speed being c ?

Thanks,

--
Boo

Creating a device which produces exactly one photon on demand (a.k.a. a
photon gun) is quite a challenge and I'm not sure one has yet been
created. So the short answer is "no".

There are single-photon-on-demand devices now. The googleable phrase
is "photon turnstile". Here is a report.

As for the original question, for gamma-ray photons there are things
called "tagged sources". This consists of an isotope (e.g. Am241)
which decays and approximately simultaneously releasing an alpha
particle (helium nucleus) and a gamma-ray photon. The isotope is
embedded in a scintillator that produces a light flash when the alpha
is released.

This is used for a source where you know exactly when the gamma-rays
are produced. The gammas are always detected at the correct time.

Creating a device which produces exactly one photon on demand (a.k.a. a
photon gun) is quite a challenge and I'm not sure one has yet been
created. So the short answer is "no".

I recall a fair amount of experimental work on a related
problem, involving driving LEDs with current sources
designed to provide a stream of antibunched electrons,
and so producing antibunched light.

Try using google with the terms "antibunch"/ed/ing and
LED and see what turns up. It's not the "one photon on demand"
requested, but might not be so far off by now, especially
if you work out some trick to gate the signal.